In recent years, a group III nitride semiconductor light emitting device has been attracting attention as a semiconductor material for light emitting devices which emit light of short wavelength. A group III nitride semiconductor is represented by the general formula AlxGayInzN (0≦x≦1, 0≦y≦1, 0≦z≦1 and x+y+z=1), and is formed on top of a substrate made of a sapphire single crystal, various kinds of oxides, or a group III-V compound, through a metal organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method), or the like.
In a typical light emitting device using a group III nitride semiconductor, an n-type semiconductor layer, a light emitting layer, and a p-type semiconductor layer are laminated in this order on top of a sapphire single crystal substrate. Since the sapphire substrate is an insulator, the device structure generally has a structure in which a positive electrode formed on top of the p-type semiconductor layer and a negative electrode formed on top of the n-type semiconductor layer are present on the same plane. Such a group III nitride semiconductor light emitting device has two types: a face up type in which a transparent electrode is used as a positive electrode to extract light from the p-type semiconductor side; and a flip chip type in which a highly reflective film of Ag or the like is used as a positive electrode to extract light from the sapphire substrate side.
The external quantum efficiency is used as an indicator for the output of such light emitting devices. When the external quantum efficiency is high, it is possible to say that the light emitting device has a high output. The external quantum efficiency is represented as the product of internal quantum efficiency and light extraction efficiency. The internal quantum efficiency refers to the proportion of energy converted into light in the light emitting layer amongst the energy of an electric current applied to the device. The light extraction efficiency refers to the proportion of light that can be extracted to the outside of the light emitting device amongst light generated in the light emitting layer. Accordingly, in order to improve the external quantum efficiency, the light extraction efficiency needs to be enhanced.
There are mainly two ways to improve the light extraction efficiency. One is a method to reduce the absorption of light by the electrode or the like formed on the light extraction surface. The other one is a method for reducing light confinement within the light emitting device occurring due to a difference in the refractive index between the light emitting device and an outside medium thereof.
In addition, when a transparent electrode is to be provided on top of a p-type semiconductor layer serving as the outermost layer so as to improve the light extraction efficiency of a light emitting device, a metallic transparent electrode made of Ni/Au or the like has been used conventionally. However, in recent years, a transparent electrode made of a translucent conductive oxide film of ITO or the like has been used instead.
One of the reasons why the transparent electrodes have been substituted from the metallic transparent electrodes made of Ni/Au or the like to the translucent conductive oxide films made of ITO or the like, as described above, is that the absorption of emitted light can be reduced by the use of translucent conductive oxide films.
In addition, as to the method for reducing light confinement within the light emitting device, a technique for forming an uneven pattern on the light extraction surface of the light emitting device can be used (for example, refer to Patent Document 1).
However, in the light emitting device in which an uneven pattern is formed on the light extraction surface by means of mechanical or chemical processing, the processing on the light extraction surface causes overloading on the semiconductor layer, leaving damage in the light emitting layer. In addition, in the light emitting device in which the semiconductor layer has been grown under such a condition that allows an uneven pattern to be formed on the light extraction surface, the crystallinity of the semiconductor layer is deteriorated, thereby making the light emitting layer defective. For this reason, when an uneven pattern is formed on the light extraction surface, although the light extraction efficiency improves, the internal quantum efficiency is lowered, causing a problem in that the light emission intensity cannot be increased.
Accordingly, instead of forming an uneven pattern on the light extraction surface, a method for forming an uneven pattern on the surface of the sapphire substrate to grow a group III nitride semiconductor layer thereon, has been proposed (for example, refer to Patent Document 2). Further, a method has been proposed for growing a GaN crystal on top of a substrate where curved convex portions have been formed as an uneven pattern on the substrate (for example, refer to Patent Document 3). Furthermore, a method has also been proposed for depositing a buffer layer on top of a translucent substrate, in which an uneven pattern has been formed, by a sputtering process using a sputtering apparatus (for example, refer to Patent Document 4).
According to such methods, the interface between the sapphire substrate and the group III nitride semiconductor layer becomes an uneven shape, and the difference in the refractive index between the sapphire substrate and the group III nitride semiconductor layer causes diffuse reflection of light in the interface, which can reduce the light confinement within the light emitting device and can improve the light extraction efficiency. In addition, due to the uneven shape formed on the surface of the sapphire substrate, it becomes possible to reduce crystal defects by taking advantage of crystals growing in the transverse direction and to improve the internal quantum efficiency.